Manufacturing device of foamed thermoplastic resin sheet

ABSTRACT

A foamed thermoplastic resin sheet manufacturing device is provided with (i) an extruder for melting and kneading a thermoplastic resin and a foaming agent to form a melted/kneaded mixture and extruding the melted/kneaded mixture, (ii) a die which is provided at a front end of said extruder and forms the melted/kneaded mixture into a sheet-like foamy thermoplastic resin material, (iii) a vacuum chamber in which the foamy thermoplastic resin material extruded through said die expands under a reduced pressure, and (iv) a facing-wall section composed of a pair of walls of said vacuum chamber which face each other in a thickness direction of the foamy thermoplastic resin material, at least one of the walls in pair being a movable wall provided so as to move in directions in which said walls approach and separate each other. By this arrangement, foamed thermoplastic resin sheets with various thicknesses can be produced.

FIELD OF THE INVENTION

The present invention relates to a foamed thermoplastic resin sheetmanufacturing device which performs expansion under a reduced pressure.

BACKGROUND OF THE INVENTION

As a conventional method for producing a foamed thermoplastic resinsheet (hereinafter sometimes referred to simply as “sheet”), amanufacturing method wherein a thermoplastic resin and a foaming agentare melted and kneaded in an extruder and extruded through a die intounder the atmospheric pressure has been well known. To obtain a highlyfoamed sheet by this manufacturing method, however, there areinconveniences that a great amount of a foaming agent is needed, andthat cells become more coarse as the sheet expands, therebydeteriorating a strength of the sheet.

As a method which has such inconveniences solved, a method in which thefoamy thermoplastic resin material extruded from an extruder is passedthrough a vacuum device so that the foamy thermoplastic resin is furtherexpanded has been practiced.

For example, the Japanese Publication for Laid-Open Patent ApplicationNo. 54215/1990 (Tokukohei 2-54215) (the Japanese Patent No. 1639854)discloses an arrangement wherein a roll-like haul-off machine isinstalled in a vacuum chamber so that the sheet-like foamy thermoplasticresin material is extruded through a die to the vacuum chamber so as toexpand and the material thus foamed is hauled by the haul-off machine.

The Japanese Examined Patent Publication 29328/1983 (The Japanese PatentNo. 1199174) discloses an arrangement in which a sealing member forensuring reduction of a pressure in a vacuum chamber is provided at anoutlet of the vacuum chamber and a roll-like haul-off machine isinstalled behind the vacuum chamber so as to haul a sheet-like foamythermoplastic resin material which is extruded through a die to thevacuum chamber thereby expanding.

Incidentally, in the present specification, a thermoplastic resinexpanded under a reduced pressure at a first expansion stage is referredto as “foamy thermoplastic resin,” and the foamed thermoplastic resinfurther expanded under the reduced pressure to a completely expandedstate, obtained after or immediately before curing, is referred to as“foamed thermoplastic resin.” A thermoplastic resin in a state of beingexpanded under a reduced pressure is to be classified as the former“foamy thermoplastic resin.”

By the conventional arrangements described above, foamed thermoplasticresin sheets with only one thickness are produced. Therefore, to producefoamed thermoplastic resin sheets with various thicknesses, a discretemanufacturing device has to be prepared for each thickness.

The manufacturing device disclosed by Tokukohei 2-54215, however, has adrawback in that installment of the haul-off machine in the vacuumchamber causes the sealing mechanism in the vacuum chamber to becomecomplicated, and makes the manufacturing device bulkier.

Further, regarding the manufacturing method disclosed by Tokukosho58-29328, since the foamed sheet, while being hauled by the haul-offmachine, pushes the sealing member at the outlet of the manufacturingdevice, cells in the foamed sheet are crushed and surfaces of the sheetare scarred. Note that the drawback in that cells of the foamed sheetare crushed is more remarkable in the case of Tokukohei 2-54215.

Therefore, though usually cells growing long in the thickness directionof the sheet are obtained in the case of expansion under a reducedpressure, such an effect of growth of cells due to pressure reduction isnot sufficiently achieved in the foregoing prior art, and cells emergingin this case are cells growing long in the direction orthogonal to thesheet thickness direction, that is, in the sheet width direction or inthe extrusion direction, like in the aforementioned case. Presence of anumber of such cells in a sheet leads to a problem that the sheet cannotbe made thicker.

SUMMARY OF THE INVENTION

The present invention was made in light of the foregoing problems, andthe object of the present invention is to provide a manufacturing deviceof a foamed thermoplastic resin sheet which is capable of manufacturingfoamed thermoplastic resin sheets with various thicknesses.

To achieve the foregoing object, a manufacturing device of a foamedthermoplastic resin sheet of the present invention is characterized bycomprising (i) an extruder for melting and kneading a thermoplasticresin and a foaming agent to form a melted/kneaded mixture, andextruding the melted/kneaded mixture, (ii) a die provided at a front endof the extruder, for forming the melted/kneaded mixture into asheet-like foamy thermoplastic resin material, (iii) a vacuum chamber inwhich the foamy thermoplastic resin material extruded through the dieexpands under a reduced pressure, and (iv) a facing-wall sectioncomposed of a pair of walls of the vacuum chamber which face each otherin a thickness direction of the foamy thermoplastic resin material, atleast one of the walls being a movable wall provided so as to move indirections in which the walls approach and separate each other.

With the foregoing arrangement, the melted/kneaded mixture of thethermoplastic resin and the foaming agent is extruded through the die byan extruding operation by the extruder, thereby becoming a sheet-likefoamy thermoplastic resin material. The foamy thermoplastic resinmaterial further expands under a reduced pressure in the vacuum chamber,thereby becoming the formed thermoplastic resin sheet.

Here, the thickness of the foamed thermoplastic resin sheet isdetermined depending on a dimension of the vacuum chamber in the foamedthermoplastic resin sheet thickness direction. In other words, thefoamed thermoplastic resin sheet expands to a thickness equivalent tothe dimension of the vacuum chamber in the foamed thermoplastic resinsheet thickness direction.

On the other hand, in the facing-wall section composed of a pair ofwalls of the vacuum chamber which face each other in the thicknessdirection of the foamy thermoplastic resin material, at least one of thewalls is a movable wall provided so as to move in directions in whichthe walls approach and separate each other. Therefore, the thickness ofthe foamed thermoplastic resin sheet in the vacuum chamber can bechanged by moving the movable wall, and as a result, the foamedthermoplastic resin sheet can be formed to various thicknesses.

Another object of the present invention is to provide a manufacturingdevice of a foamed thermoplastic resin sheet which is capable ofproducing a thick sheet whose surface condition is excellent and whosegood foaming state under a reduced pressure is maintained therebyensuring a high foaming ratio.

To achieve the foregoing object, a manufacturing device of a foamedthermoplastic resin sheet of the present invention, arranged as above,is further arranged as follows.

Namely, a sealing member is provided at least on the movable wall in thefacing-wall section, at an outlet of the vacuum chamber, so as to seal aspace between the wall and a foamed thermoplastic resin sheet which isobtained as a result of expansion of the foamy thermoplastic resinmaterial under a reduced pressure and comes out of the vacuum chamber.

The foregoing arrangement ensures that a set desired pressure is kept inthe vacuum chamber, without a great force applied from the sealingmembers to the foamy thermoplastic resin material and the foamedthermoplastic resin sheet, at all times since the start of pressurereduction.

For example, the gap between the walls in pair of the facing-wallsection is narrowed in a period prior to pressure reduction in thevacuum chamber, i.e., when expansion of the sheet-like foamythermoplastic resin material due to pressure reduction does not yetstart and the material has a small thickness, whereas the gap is widenedafter the pressure reduction in the vacuum chamber starts therebycausing the sheet-like foamy thermoplastic resin material to expand dueto the reduced pressure and have a greater thickness, that is, when thesheet-like foamy thermoplastic resin material becomes the foamedthermoplastic resin sheet. Therefore, before and after the start of thepressure reduction in the vacuum chamber, the respective relationshipsbetween the walls and the sheet-like foamy thermoplastic resin materialor the foamed thermoplastic resin sheet, that is, the respectiveposition relationship between the sealing members and the sheet-likefoamy thermoplastic resin material or the foamed thermoplastic resinsheet in the thickness direction of the sheet-like material (or sheet)can be maintained substantially unchanged. This allows the sealingmembers to be made of a soft and flexible material, and the sealingmembers thus formed by no means scar the surfaces of the foamedthermoplastic resin sheet, nor crush cells in the foamed thermoplasticresin sheet.

In contrast, by an arrangement in which the facing-wall section does nothave a movable wall, it is required to make the sealing members of amaterial with a relatively high rigidity so as to maintain a desirablyreduced pressure in the vacuum chamber before and after a change of thethickness of the sheet due to expansion under the reduced pressure. Inthis case, an excessively great force is applied from the sealingmembers to the foamed thermoplastic resin sheet with a greater thicknessdue to expansion under the reduced pressure, whereby the surfaces of thefoamed thermoplastic resin sheet tend to be scarred by the sealingmembers, or cells in the sheet tend to be crushed.

Consequently, with a simple arrangement of the present invention inwhich sealing members are provided as sealing means for maintaining thereduced pressure in the vacuum chamber, a sheet whose surface is notscarred by the sealing members and which maintains a good foaming stateunder a reduced pressure thereby having a high foaming ratio and beingformed thick can be easily obtained.

For a fuller understanding of the nature and advantages of theinvention, reference should be made to the ensuing detailed descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a schematic cross-sectional view illustrating anarrangement of a foamed thermoplastic resin sheet manufacturing devicein accordance with an embodiment of the present invention.

FIG. 1(b) is a plan view of the same.

FIG. 2 is a schematic cross-sectional view taken along an A—A arrow linein FIG. 1(b).

FIG. 3(a) is a schematic vertical cross-sectional view illustrating ashape of POROUS ELECTROCAST which is adaptable as a porous material usedin an expanding zone and a cooling zone shown in FIG. 1(a), as well asillustrating how a surface of the same is made uneven.

FIG. 3(b) is a schematic vertical cross-sectional view illustratinganother example of the arrangement shown in FIG. 3(a).

FIG. 4(a) is a schematic vertical cross-sectional view illustratinganother example of a manner how to make the surface uneven as shown inFIG. 3(a).

FIG. 4(b) is a schematic vertical cross-sectional view illustratingstill another example of a manner how to make the surface uneven asshown in FIG. 3(a).

FIG. 5(a) is a schematic vertical cross-sectional view illustrating astate of the manufacturing device shown in FIG. 1(a) upon initializationof extrusion of a sheet-like foamy thermoplastic resin material.

FIG. 5(b) is a schematic vertical cross-sectional view illustrating astate subsequent to the state shown in FIG. 5(a), prior to pressurereduction of the vacuum chamber.

FIG. 6 is a schematic vertical cross-sectional view illustrating anotherexample of the manufacturing device shown in FIG. 1(a), in which a dielip projects into the vacuum chamber.

FIG. 7 is a schematic cross-sectional view illustrating an arrangementof a foamed thermoplastic resin sheet manufacturing device in accordancewith another embodiment of the present invention.

FIG. 8(a) is a schematic vertical cross-sectional view illustrating astate of the manufacturing device shown in FIG. 7 upon initialization ofextrusion of a sheet-like foamy thermoplastic resin material.

FIG. 8(b) is a schematic vertical cross-sectional view illustrating astate which is subsequent to the state shown in FIG. 8(a) and prior topressure reduction of the vacuum chamber.

FIG. 9 is a schematic vertical cross-sectional view illustrating anarrangement of a foamed thermoplastic resin sheet manufacturing devicein accordance with still another embodiment of the present invention.

FIG. 10(a) is a schematic vertical cross-sectional view illustrating anarrangement of a foamed thermoplastic resin sheet manufacturing devicein accordance with still another embodiment of the present invention.

FIG. 10(b) is a plan view of the same.

FIG. 11(a) is a schematic vertical cross-sectional view illustrating anarrangement of a foamed thermoplastic resin sheet manufacturing devicein accordance with still another embodiment of the present invention.

FIG. 11(b) is a plan view of the same.

FIG. 12 is an explanatory view illustrating a method for measuring celldiameters of a foamed thermoplastic resin sheet produced by themanufacturing device shown in FIG. 1(a) in accordance with an example ofthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[First Embodiment]

The following description will explain an embodiment of the presentinvention while referring to FIGS. 1 through 6.

A manufacturing device of the foregoing foamed thermoplastic resin sheet6 is equipped with an extruder 1, a head part 2, a die 3, a vacuumchamber 4, and a haul-off machine 5, as shown in FIG. 1(a). The vacuumchamber 4 is installed inside a molding section 9.

The extruder 1 melts and kneads a foaming agent and a thermoplasticresin at a temperature previously set suitable to the thermoplasticresin and the foaming agent used, and extrudes the mixture obtainedtoward the head part 2. It is preferable that the extruder 1 has asingle shaft structure, in the case where the foaming agent and thethermoplastic resin are kneaded at a low temperature.

The temperature of the extruder 1 is set, for example, in the case wherethe thermoplastic resin used is polypropylene resin, so that atemperature of a melt of polypropylene resin (kneaded material) as afoamy thermoplastic resin material is not higher than 180° C. in thevicinity of an outlet of the extruder 1. This is because outgassingoccurs when the temperature of the melt of the polypropylene resinexceeds 180° C.

The head part 2 is positioned at the outlet of the extruder 1, and ascreen mesh usually used for molding by extrusion is used. In the casewhere, however, the thermoplastic resin used is a resin characterized byremarkable temperature rise when being shorn, a screen mesh is not used.

The die 3 is a sheet die for processing the foamy thermoplastic resinmaterial extruded from the extruder 1 through the head part 2 into asheet form. Generally, the sheet die is arranged so that temperature andpressure are adjustable. The die 3 has a die lip 3 a as an outletthrough which the resin is discharged.

The haul-off machine 5 positioned on a side to an outlet of the vacuumchamber 4 so as to haul the foamed thermoplastic resin sheet 6 throughthe outlet of the vacuum chamber 4. The haul-off machine 5 is composedof not less than one pair of rolls 5 a facing each other so as to becapable of nipping the foamed thermoplastic resin sheet 6 therebetween.The rolls 5 a are movably provided in directions such that a gaptherebetween is narrowed and widened. For example, the rolls 5 a aremovably provided so as to move as a movable upper wall 12 and a movablelower wall 13 which will be described later move. Incidentally, eachroll 5 a is preferably arranged so that a temperature thereof isadjustable by cooled water. In the place of the roll-type haul-offmachine 5, another type haul-off machine (for example, a belt-type one)conventionally used in manufacture of resin sheets may be used.

A hauling speed of the haul-off machine 5 should be appropriately setaccording to a foaming ratio, a thickness, a resin composition, and thelike of the foamed thermoplastic resin sheet 6, but usually it is set to1 to 3 m/min.

The vacuum chamber 4 is used for further expanding the sheet-like foamythermoplastic resin material (hereinafter referred to as sheet-likefoamy thermoplastic resin material 6 a, so as to be distinguished fromthe foamed thermoplastic resin sheet 6 in the completely expanded stage)extruded from the die 3 under a reduced pressure. The vacuum chamber 4has such a structure as is capable of cooling the sheet-like foamythermoplastic resin material 6 a after further expanding the sheet-likefoamy thermoplastic resin material 6 a.

An inlet side part of the vacuum chamber 4 is an expanding zone 7 inwhich the sheet-like foamy thermoplastic resin material 6 a extrudedfrom the die 3 is expanded under a reduced pressure, and an outlet sidepart thereof is a cooling zone 8 in which the sheet-like foamythermoplastic resin material 6 a expanded in the expanding zone 7 iscooled so as to be cured. Incidentally, there is no need to strictlyseparate the expanding zone 7 and the cooling zone 8 with use of aseparating board or the like. The expanding zone 7 has a pressure nothigher than that of the cooling zone 8, and also has a function ofcooling the sheet-like foamy thermoplastic resin material 6 a afterexpanding it. The cooling zone 8 is a zone in which the sheet-like foamythermoplastic resin material 6 a is cured.

Evacuation of the vacuum chamber 4 is carried out by a vacuum pump 17connected to the expanding zone 7, through inner walls 14 having vacuumports, which are provided in the expanding zone 7 and the cooling zone8. The inner walls 14 will be described later. Adjustment of the reducedpressure is conducted by a pressure regulator 15 and a vacuum breaker16. In adjustment of the reduced pressure, the pressure regulator 15 andthe vacuum breaker 16 may be used in combination, or alternatively, oneof them may be used. Here, the expanding zone 7 and the cooling zone 8are evacuated by one evacuating pump 17 at the same time, butalternatively, another evacuating pump 17 may be provided to the coolingzone 8 so that pressures of the zones 7 and 8 are independentlyadjusted. In addition, it is preferable that each evacuating line isequipped with a pressure adjusting valve so that the reduced pressurecan be adjusted. Applicable as the pressure adjusting valve is agenerally-used type, such as a type which controls an inlet pressure ofthe evacuating pump 17 by changing an aperture thereof with use of apressure detector and a pressure transmitter, or that of a type whichper se controls the pressure by monitoring a pressure gauge.

Regarding pressure in the vacuum chamber 4, a differential pressure ofabout 200 mmHg (a difference from the atmospheric pressure) is requiredin the case where the thermoplastic resin used is for examplepolypropylene resin. The differential pressure is preferably set notless than 300 mmHg, or more preferably 350 to 700 mmHg. An optimalpressure, however, varies depending on the thermoplastic resin and thefoaming agent used, and also varies with a desired foaming ratio of thefoamed thermoplastic resin sheet 6.

The following description will explain the molding section 9 and thevacuum chamber 4 in more detail.

As shown in FIG. 1(a) and FIG. 2 which is a view (the sheet-like foamythermoplastic resin material 6 a is omitted) of a cross section takenalong an A—A arrow line in FIG. 1(b), the molding section 9 includes themovable upper wall 12 and the movable lower wall 13 which are verticallymovable, in a space enclosed by outer walls 11 in a box-like form. Thespace enclosed by the movable upper and lower walls 12 and 13 and theouter walls 11 constitutes the vacuum chamber 4. In the presentembodiment, a wall surface 12 a which is a lower surface of the movableupper wall 12 and a wall surface 13 a which is an upper surface of themovable lower wall 13 are plane. The movable upper wall 12 and themovable lower wall 13 have the inner walls 14 on their sides to thevacuum chamber 4, and surfaces of the inner walls 14 constitute theaforementioned wall surfaces 12 a and 13 a, respectively.

The molding section 9 is equipped with a movable wall driving device 23for moving the movable upper wall 12 and the movable lower wall 13. Themovable wall driving device 23 has a plurality of screws 24 whichvertically pierce the outer wall 11, and ends of the screws 24 are fixedto an upper surface of the movable upper wall 12. Portions of the screws24 projecting from the outer wall 11 are fit to internal threads (notshown) formed in sprockets 25. The sprockets 25 are rotatably providedon an outer surface of the outer wall 11. On the outer surface of theouter wall 11, there is provided a handle 26 which is rotatable, and asprocket 27 is made rotatable by the handle 26. To the sprockets 25 and27, a belt 28 with teeth is applied, as shown in FIG. 1(b). The belt 28may be substituted by a chain.

On the lower side of the movable lower wall 13, likewise, there areprovided screws 24, sprockets 25, a sprocket 27, and a belt 28. On thelower side, however, no handle 26 is provided, and rotation of thehandle 26 on the upper side is transmitted to the sprocket 27 on thelower side by a drive transmitting mechanism not shown. Alternatively,however, an independent movable wall driving device 23 identical to thaton the upper side may be provided on the lower side as well, so that themovable upper wall 12 and the movable lower wall 13 are independentlymoved by the handles 26, respectively, i.e., by the movable drivingdevices 23, respectively.

With the aforementioned arrangement, the movable upper wall 12 and themovable lower wall 13 are simultaneously moved upward/downward byrotating the handle 26. In this case, the movable upper wall 12 and themovable lower wall 13 are moved in opposite directions. Thus, a gapbetween the wall surface 12 a of the movable upper wall 12 and the wallsurface 13 a of the movable lower wall 13, that is, a height of thevacuum chamber 4 equivalent to a thickness of the foamed thermoplasticresin sheet 6 is adjustable.

Incidentally, movement of the movable upper wall 12 and the movablelower wall 13 is preferably performed in a state in which the movableupper and lower walls 12 and 13 slant in neither the extrusion directionnor the width direction of the foamed thermoplastic resin sheet 6, beingparallel with each other at all times.

Furthermore, in the present embodiment, the wall surface 12 a of themovable upper wall 12 and the wall surface 13 a of the movable lowerwall 13 move equal distances, respectively, with respect to a centralposition of the foamed thermoplastic resin sheet 6 in the thicknessdirection at the die lip 3 a.

Furthermore, an arrangement of the upper wall driving device 23 is notlimited to the foregoing screw type. Known arrangements, such as anarrangement in which hydraulic cylinders are used, may be adapted. Theforegoing screw type is suitable to a small-scale manufacturing device,while the hydraulic cylinder type is suitable to a large-scalemanufacturing device.

Furthermore, in the present embodiment, the movable upper and lowerwalls 12 and 13 are provided, but alternatively, only one of them may bemovably provided.

On edges of the movable upper and lower walls 12 and 13 on the vacuumchamber 4 outlet side, blade-like sealing members 31 are provided,respectively. The sealing members 31 are provided on the movable upperand lower walls 12 and 13, respectively, for example, from a right orleft end to the other end on each wall edge. The sealing members 31 sealthe vacuum chamber 4 so that reduction of pressure in the vacuum chamber4 is ensured. In a state in which the sheet-like foamy thermoplasticresin material 6 a or the foamed thermoplastic resin sheet 6 are presentbetween the sealing members 31, the sealing members 31 curve or bend ina transport direction of the same. The sealing members 31 are soft andflexible, made of, for example, rubber.

Another sealing member 31 may also be provided in the thicknessdirection of the foamed thermoplastic resin sheet 6 on both sides to theedges of the sheet 6, in addition to the foregoing positions. Thesealing members 31 in the thickness direction are fixed to the outerwalls 11. Further, the sealing members 31 may be provided either on anupper side or on a lower side to the sheet-like foamy thermoplasticresin material 6 a or the foamed thermoplastic resin sheet 6, in a widthdirection.

In the vacuum chamber, the expanding zone 7 is a zone in which thesheet-like foamy thermoplastic resin material 6 a extruded by the die 3is to be further expanded with the temperature thereof adjusted suitablefor foaming. The pressure in the expanding zone 7 is reduced. Theexpanding zone 7 is instantaneously widened to a width W₂ correspondingto the gap between the wall surfaces 12 a and 13 a, from a thicknesscorresponding to the aperture thickness of the die lip 3 a, duringmanufacture of the foamed thermoplastic resin sheet 6.

Evacuation of the expanding zone 7 is carried out through a memberhaving vacuum ports, which is provided on a part or a whole of wallssurrounding the expanding zone 7.

The evacuation may be carried out by evacuating air in the thicknessdirection, or in the direction orthogonal to the thickness direction (inthe width direction), or in both the directions. To evacuate in thethickness direction is preferable.

It is further preferable that the movable upper wall 12 have vacuumports with a diameter of 20 mm or smaller each on end portions thereofin the width direction so that the pressure is reduced in the thicknessdirection. In the case where the diameter of the vacuum ports is morethan 20 mm, melted resin tends to clog up, thereby sometimes causing thehauling operation of the foamed thermoplastic resin sheet 6 by thehaul-off machine 5 to stop.

In the present embodiment, the movable upper and lower walls 12 and 13are made of a material having the vacuum ports. As such a material, aporous material such as a sintered alloy or a porous electrocast shellis suitably adapted.

An example of the porous electrocast shell as one of the porousmaterials is PORASU DENCHU (POROUS ELECTROCAST, Japanese registeredtrademark), and FIGS. 3(a) and 3(b) schematically illustrate crosssections of the inner wall 14 made of POROUS ELECTROCAST. In POROUSELECTROCAST, a vent hole H broadens toward a reverse side. Therefore, ithas a property of being hardly clogged up, and has a low gas evacuationresistance. POROUS ELECTROCAST is an electrocast type which causes metalreversing by plating a model with a metal such as nickel.

The inner wall 14 shown in FIG. 3(b) has a thicker surface portion ascompared with that of the inner wall 14 shown in FIG. 3(a). Therefore,the surface thereof is more easily processed, and a pressure resistancethereof increases. The number of ports of POROUS ELECTROCAST is usually3/cm² to 7/cm², and preferably, 3/cm² to 5/cm². If the number is greaterthan the above, a strength gradually decreases.

It is required to form each vacuum port of the inner wall 14 not greaterthan 100 μm, preferably not greater than 50 μm, or more preferably notgreater than 30 μm. In the case where the vacuum ports are great,additives, melted resin, and decomposited resin of the thermoplasticresin used tend to clog up the vacuum ports of the inner walls 14, and alarge-scale vacuum pump 17 becomes needed to keep a desired reducedpressure.

Further, each inner wall 14 positioned in the expanding zone 7 is keptto a desired temperature by cooled water running through a cooled waterpath (temperature adjusting means, cooling medium path) 18 which isburied in the inner wall 14. In the case where the inner walls 14 aremade of a metal with a high thermal conductivity, a great cooling effectcan be achieved. The cooled water path 18 may be one line to cool thewhole expanding zone 7. To keep good the foaming state of the sheet-likefoamy thermoplastic resin material 6 a, however, it is preferable that aplurality of independent lines are juxtaposed in the extrusion directionof the sheet-like foamy thermoplastic resin material 6 a.

Incidentally, means for adjusting the temperature of the expanding zone7 is not particularly limited, provided that it is capable of adjustingthe temperature. For example, an arrangement in which air is blown intothe expanding zone 7 is acceptable. In the case where this arrangementis adapted, it is possible to maintain the reduced pressure in thevacuum chamber 4 as a whole by carrying out vacuum drawing whichoverwhelms the air blowing.

Here, since the whole, or substantially whole, surfaces of thesheet-like foamy thermoplastic resin material 6 a is brought intocontact with the inner walls 14 in the expanding zone 7, the temperatureof the inner walls 14 is fully conducted to the sheet-like foamythermoplastic resin material 6 a. As a result, a temperature adjustmenteffect is improved.

In the foregoing arrangement of the expanding zone 7, the sheet-likefoamy thermoplastic resin material 6 a is dragged while being in contactwith the inner walls 14 in the expanding zone 7. Therefore, if a contactarea thereof is large, the sheet 6 a tends to be scarred on itssurfaces, as well as smooth hauling of the sheet 6 a cannot be achieved.

To solve these problems, a number of small protuberances 21 are formedon surfaces of the inner walls 14 in the expanding zone 7, that is, onthe wall surfaces 12 a and 13 a in the expanding zone 7, as shown inFIGS. 3(a) and 3(b). The protuberances 21 shown in FIGS. 3(a) and 3(b)have curving surfaces and are independently formed. By making the wallsurfaces 12 a and 13 a uneven with the protuberances 21, an area of thewall surfaces 12 a and 13 a in contact with the sheet-like foamythermoplastic resin material 6 a (for example, the sheet-like foamypolypropylene resin material) or the foamed thermoplastic resin sheet 6is reduced, whereby smooth sliding of the sheet-like foamy thermoplasticresin material 6 a or the foamed thermoplastic resin sheet 6 isachieved.

The area of the wall surfaces 12 a and 13 a in contact with thesheet-like foamy thermoplastic resin material 6 a is preferably reducedto not less than 10 percent and not more than 80 percent the area in thecase where the protuberances do not exist. In the case where the area ofthe wall surfaces 12 a and 13 a in contact with the sheet-like foamythermoplastic resin material 6 a is less than 10 percent, it is toosmall to ensure that the sheet-like foamy thermoplastic resin material 6a is sufficiently cooled. In the case where the contact area is greaterthan 80 percent, the hauling of the sheet-like foamy thermoplastic resinmaterial 6 a tends to become difficult.

Furthermore, it is more preferable that the protuberances 21 are platedwith, for example, polytetrafluoroethylene (Teflon). This makes thesliding of the sheet-like foamy thermoplastic resin material 6 asmoother, thereby decreasing probability that the surfaces thereof arescarred.

Patterns of the protuberances 21 shown in FIGS. 3(a) and 3(b) are merelyexamples, and there is no particular limitation on the pattern of theprotuberances. The crepe pattern, or the crape pattern, for example, issuitably applied. The formation of protuberances in such a pattern andthe plating of the same are more preferably carried out with respect to,not only the wall surfaces 12 a and 13 a of the expanding zone 7, butalso wall surfaces of the outer walls 11 (see FIG. 2) enclosing theexpanding zone 7. Alternatively, however, they may be carried out withrespect to only a part of them. In the case where a porous material isused for drawing a vacuum, it is preferable that the formation ofprotuberances in a pattern and the plating of the same are carried outwith respect to both the metal surfaces and surfaces of the porousmaterial constituting inner wall surfaces of the expanding zone 7, butthe formation of protuberances may be carried out with respect to onlyeither the metal surfaces or the porous material.

FIGS. 3(a) and 3(b) show arrangements in which the protuberances 21differing in size are provided, but the protuberances 21 formed to auniform size may be provided, as shown in FIGS. 4(a) and 4(b). Regardinga shape of each protuberance 21, the protuberance 21 preferably has acurved surface (see FIGS. 3(a) and 4(a)). The shape is, however, notlimited to this, and it may be a pyramid, or a cone (see FIG. 4(b)).

An apparent friction coefficient k between the sheet-like foamythermoplastic resin material 6 a and the wall surfaces 12 a and 13 a ispreferably controlled so as to become not more than 0.4, by forming theprotuberances in a desired pattern and plating them with, for example,polytetrafluoroethylene (Teflon). Here, the apparent frictioncoefficient k is defined by the following expression:

(DIFFERENTIAL PRESSURE)×(AREA OF WALL SURFACE 12a(OR 13a))×k=HAULINGFORCE

where the differential pressure is a differential pressure betweenpressure in the vacuum chamber 4 and the atmospheric pressure.

The apparent friction coefficient k is preferably not greater than 0.35,or more preferably not greater than 0.32. This is because that in thecase where the apparent friction k is greater than 0.4, the hauling ofthe foamed thermoplastic resin sheet 6 by the haul-off machine 5 tendsto stop.

On the other hand, the cooling zone 8 is a zone in which the foamedthermoplastic resin sheet 6 expanded in the thickness direction in theexpanding zone 7 is cooled so as to be cured. The structure for drawinga vacuum in the cooling zone 8 is identical to that in the expandingzone 7, and air inside is evacuated through a member with vacuum ports.As the material with the vacuum ports, a porous material is desirablehere as well, and conditions regarding an aperture of the vacuum ports,arrangement of the same, and suitable materials for forming the member,and the like, are the same as those described relating to the expandingzone 7. The number of the vacuum ports, however, may be smallerconcerning the cooling zone 8 than that concerning the expanding zone 7.In the case where POROUS ELECTROCAST is used as a porous material, thenumber of pores of POROUS ELECTROCAST is not particularly limited, butit is preferably not more than 3/cm².

The cooling zone 8 preferably has a lower pressure than the pressure inthe expanding zone 7 (has a pressure close to the atmospheric pressure).In this case, an effect of facilitating the hauling of the foamedthermoplastic resin sheet 6 by the haul-off machine 5 can be achieved.

In the cooling zone 8 as well, the surfaces of the foamed thermoplasticresin sheet 6 are completely or partially brought into contact with theinner walls 14 in the cooling zone 8, thereby allowing the temperatureof the inner walls 14 to be fully transmitted to the foamedthermoplastic resin sheet 6. As a result, a high cooling effect can beachieved.

Further, in the cooling zone 8 as well, like in the expanding zone 7,the foamed thermoplastic resin sheet 6 is dragged. Therefore, thesurfaces of the inner wall surfaces of the cooling zone 8 are madeuneven, with protuberances 21 being formed on the inner wall surfaces.More preferably, the protuberances are plated.

Incidentally, means for adjusting the temperature of the cooling zone 8is not particularly limited, provided that it is capable of adjustingthe temperature. An arrangement in which temperature adjustment isconducted with use of a cooling medium with a set desired temperature,an arrangement in which air is blown into the cooling zone 8, or thelike is acceptable. In the case where the arrangement in which air isblown thereto is adapted, it is possible to maintain the reducedpressure in the vacuum chamber 4 as a whole, by carrying out vacuumdrawing which overwhelms the air blowing. In the case where theforegoing arrangement is adapted, particularly, friction of the foamedthermoplastic resin sheet 6 with the inner wall surfaces is reduced,whereby the hauling of the foamed thermoplastic resin sheet 6 is furtherfacilitated.

At the outlet of the cooling zone 8, it is preferable that the foamedthermoplastic resin sheet 6 has a central temperature of not higher than50° C. which is a central temperature of the sheet-like foamythermoplastic resin material 6 a at the outlet of the die 3. By sodoing, it is possible to maintain the cells grown in the thicknessdirection in the formed thermoplastic resin sheet 6 under a reducedpressure.

In the case where the foamed thermoplastic resin sheet manufacturingdevice arranged as described above is used, the foamed thermoplasticresin sheet 6 can be obtained through the following procedure.

First, the foaming agent and the thermoplastic resin are melted andkneaded by the extruder 1, and thereafter, the kneaded mixture isextruded by the die 3 into a sheet form, thereby being formed into thesheet-like foamy thermoplastic resin material 6 a. Here, themanufacturing device is in a state in which, as shown in FIG. 5(a), themovable upper and lower walls 12 and 13 are positioned by the movablewall driving device (driving means) 23 at upper and lower positions,respectively, so that a gap between the wall surfaces 12 a and 13 a iswider than the thickness of the sheet-like foamy thermoplastic resinmaterial 6 a.

The sheet-like foamy thermoplastic resin material 6 a extruded from thedie 3 reaches the haul-off machine 5 through the vacuum chamber 4,becoming ready to be hauled by the haul-off machine 5. Here, the rolls 5a facing each other are in a state in which a gap therebetween isnarrowed to the thickness of the sheet-like foamy thermoplastic resinmaterial 6 a.

Subsequently, the sheet-like foamy thermoplastic resin material 6 a iscontinuously extruded while the movable upper and lower walls 12 and 13are moved so that the gap between the wall surfaces 12 a and 13 a isreduced to W₁ by the movable wall driving device 23, as shown in FIG.5(b). The gap W₁ is equal to a thickness of the sheet-like foamythermoplastic resin material 6 a extruded from the die 3 when the vacuumchamber 4 is not subject to pressure reduction. In this state, ends ofthe sealing members 31 sufficiently reach the surfaces of the sheet-likefoamy thermoplastic resin material 6 a and covers the outlet of thevacuum chamber 4, thereby ensuring the state in which the vacuum chamber4 is ready to be subject to pressure reduction. Here, the sealingmembers 31 are curved or bent in a direction of transport of thesheet-like foamy thermoplastic resin material 6 a, in contact with thesurfaces of the sheet-like foamy thermoplastic resin material 6 a.

Thereafter, the pressure in the vacuum chamber 4 is reduced so as tobecome lower than the atmospheric pressure by not less than 100 mmHg,while the movable upper and lower walls 12 and 13 are moved until a gapbetween the wall surfaces 12 a and 13 a becomes equal to the width W₂,as shown in FIG. 1(a). The gap W₂ is set equal to the desired thicknessof the foamed thermoplastic resin sheet 6 to be produced, and it can bearbitrarily changed. Incidentally, the pressure is preferably reduced bynot more than 700 mmHg. Under the foregoing set conditions, the foamedthermoplastic resin sheet 6 can be smoothly hauled from the vacuumchamber 4.

With the foregoing pressure reducing operation, the sheet-like foamythermoplastic resin material 6 a further expands while going through theexpanding zone 7, thereby becoming the foamed thermoplastic resin sheet6. The foamed thermoplastic resin sheet 6 is cooled to be cured when theadjoining cooling zone 8, and subsequently hauled by the haul-offmachine 5. Incidentally, in the case where the foamed thermoplasticresin sheet 6 is continuously produced, the vacuum chamber 4 is subjectto pressure reduction, while the gap between the wall surfaces 12 a and13 a is fixed to W₂.

As described above, since the present manufacturing device is arrangedso that the gap W₂ between the wall surfaces 12 a and 13 a when thefoamed thermoplastic resin sheet 6 is continuously manufactured isarbitrarily set by the movable wall driving device 23, it is possible toproduce the foamed thermoplastic resin sheet 6 with a desired thickness.Therefore, the manufacturing device can be used for forming the foamedthermoplastic resin sheet 6 to various thicknesses. Thus, themanufacturing device has high flexibility.

Furthermore, by the movable wall driving device 23, the gap between thewall surface 12 a of the movable upper wall 12 and the wall surface 13 aof the movable lower wall 13 is narrowed in a period prior to pressurereduction in the vacuum chamber 4, i.e., when expansion of thesheet-like foamy thermoplastic resin material 6 a due to pressurereduction does not yet start and the material 6 a has a small thickness,whereas the gap is widened after the pressure reduction in the vacuumchamber 4 starts thereby causing the sheet-like foamy thermoplasticresin material 6 a to expand due to the reduced pressure and have agreater thickness, that is, when the sheet-like foamy thermoplasticresin material 6 a becomes the foamed thermoplastic resin sheet 6.Therefore, before and after the start of the pressure reduction in thevacuum chamber 4, the respective relationships between the wall surfaces12 a and 13 a and the sheet-like foamy thermoplastic resin material 6 aor the foamed thermoplastic resin sheet 6, that is, the respectiveposition relationship between the sealing members 31 and the sheet-likefoamy thermoplastic resin material 6 a or the foamed thermoplastic resinsheet 6 in the thickness direction of the sheet-like material 6 a (orsheet 6) can be maintained substantially unchanged. This allows thesealing members 31 to be made of a material with high flexibility, aswell as to be formed to a simple structure. For example, it may beformed to a blade-like shape by using a material such as rubber.

In other words, in the present embodiment, there is no need to make thesealing members 31 of a material with a high rigidity or to providesealing means of a complicated structure. Doing so is usually requiredfor reducing that the pressure in the vacuum chamber 4 in the case wherewide gaps exist between the sheet-like foamy thermoplastic resinmaterial 6 a and the wall surfaces 12 a and 13 a respectively before thepressure reduction in the vacuum chamber 4, as is the case with anarrangement in which the wall surfaces 12 a and 13 a are unmovable.

With the foregoing arrangement of the present embodiment, such a sealingstate of the vacuum chamber 4 that the pressure reduction is enabled ismaintained by the sealing members 31, while the sealing members 31 areprevented from, with a pressing force thereof, scarring the surfaces ofthe foamed thermoplastic resin sheet 6, or crushing cells in the foamedthermoplastic resin sheet 6. As a result, the foamed thermoplastic resinsheet 6 is formed without difficulty to have a good surface conditionand to fully enjoy an effect of cell growth due to pressure reductionthereby having the cells therein efficiently grown up in the thicknessdirection.

Incidentally, the manufacturing device of the present embodiment may bearranged so that the die lip 3 a of the die 3 projects to an inlet partof the vacuum chamber 4, as shown in FIG. 6. This arrangement ensuresprevention of such a problem that, in a state as shown in FIG. 5(b) inwhich the gap between the wall surfaces 12 a and 13 a is narrowed, apart of the sheet-like foamy thermoplastic resin material 6 a gets intoa crack between the movable upper or lower wall 12 or 13 and the die 3,thereby hindering transport of the sheet-like foamy thermoplastic resinmaterial 6 a.

Incidentally, a thermoplastic resin which can be used as a material ofthe foamed thermoplastic resin sheet 6 is not particularly limited, andany resin which is usually used as extrusion molding compound orinjection molding compound is adaptable. Examples of such resins includepolyolefin resin such as polyethylene and polypropylene, polystyrene,polyvinyl chloride, polyamide, acrylic resin, polyester, polycarbonate,and a copolymer of any ones of these resins. It is preferable thatpolypropylene resin is used as the thermoplastic resin. Thepolypropylene resin may be any one among a homopolymer, a block polymer,and a random polymer. Further, it may be a mixture of the same withanother olefin resin. In this case, the polyolefin to be mixed ispreferably a polyolefin with the number of carbon atoms of not more than10, such as polyethylene or polybutene, and polyethylene resin is mostpreferable among them. In the case where the polypropylene resin andanother polyolefin resin are mixed, a content of the polypropylene isset to 50 wt %.

An example of more preferable polypropylene resin is a propylene polymerwhose melt strength is improved. Such a propylene polymer can beobtained by a method of polymerizing components differing in molarweight at multiple stages, a method of using a specific catalyst, amethod of applying a post-processing operation such as crosslinking to apropylene polymer, or the like. Among these methods, the method ofpolymerizing components differing in molar weight at multiple stages ispreferable from a viewpoint of productivity.

The foregoing resins may contain any generally-used additive, includinga filler such as talc, a pigment, an antistatic agent, an antioxidant,and the like. No specific limitation is on the foaming agent applied tothe present invention, and any foaming agent, a physical foaming agentor a chemical foaming agent, can be used.

[Second Embodiment]

The following description will explain another embodiment of the presentinvention while referring to FIGS. 7 and 8. The members having the samestructure (function) as those in the above-mentioned embodiment will bedesignated by the same reference numerals and their description will beomitted.

A foamed thermoplastic resin sheet manufacturing device of the presentembodiment is arranged so that an upper wall and a lower wall formingthe vacuum chamber 4 of the molding section 9 are composed of a fixedupper wall 41 and a fixed lower wall 42 in a front part and the movableupper wall 12 and the movable lower wall 13 in a rear part. A spacebetween a wall surface 41 a of the fixed upper wall 41 and a wallsurface 42 a of the fixed lower wall 42 which face each otherconstitutes the expanding zone 7. The wall surfaces 41 a and 42 agradually curve or slant so that a gap therebetween is widened from aninlet to an outlet of the expanding zone 7. A gap between the wallsurfaces 41 a and 42 a is set to W₁ at the inlet, while it is set to W₃at the outlet. The gap W₃ is set, for example, equal to a thickness of afoamed thermoplastic resin sheet 6 which is thinnest amongfrequently-produced foamed thermoplastic resin sheets 6 differing inthickness. Therefore, in this case, when the foamed thermoplastic resinsheet 6 which is thinnest is produced, the gap W₃ becomes equal to thegap W₂. Note that, as described above, the gap W₂ is equivalent to adesired thickness of the foamed thermoplastic resin sheet 6 to beformed.

Placed under a reduced pressure in the expanding zone 7, the sheet-likefoamy thermoplastic resin material 6 a extruded from the die 3 expandsgradually in the thickness direction to a shape determined by the wallsurfaces 41 a and 42 a. Then, it has a thickness equal to the gap W₃ atthe end of the expanding zone 7, and has a thickness equal to the gap W₂at the inlet of the cooling zone 8.

A space between the wall surfaces 12 a and 13 a of the movable upper andlower walls 12 and 13 which face each other constitutes the cooling zone8. The wall surfaces 12 a and 13 a are plane and are movably provided soas to approach/separate to/from each other, driven by the movable walldriving device 23, as described above.

The movement of the movable upper and lower walls 12 and 13, themovement of the sealing members 31, the pressure reducing operation, andthe like during manufacture of the foamed thermoplastic resin sheet 6are identical to those in the first embodiment, and the state shown inFIG. 7 corresponds to the state shown in FIG. 1(a), while the statesshown in FIGS. 8(a) and 8(b) correspond to the states shown in FIGS.5(a) and 5(b), respectively.

With the present manufacturing device in which the gap between the wallsurfaces 41 a and 42 a is gradually widened, the sheet-like foamythermoplastic res in material 6 a extruded from the die 3 is smoothlyguided, and hauled by the haul-off machine 5 in a good condition.Furthermore, effects achieved by that the wall surfaces 12 a and 13 aare movable and that the sealing members 31 are provided are identicalto those described above.

[Third Embodiment]

The following description will explain still another embodiment of thepresent invention while referring to FIGS. 9 and 10. The members havingthe same structure (function) as those in the above-mentionedembodiments will be designated by the same reference numerals and theirdescription will be omitted.

The manufacturing device shown in FIG. 9 is equipped with a temperatureadjusting zone 51 in the molding section 9, before the vacuum chamber 4.A fixed upper wall 52 and a fixed lower wall 53, a space between whichconstitutes the temperature adjusting zone 51, have inner walls 57,respectively, which are made of a metal with a high thermalconductivity. Inside or outside the inner walls 57, there are providedtemperature adjusting medium paths 54 through which a temperatureadjusting medium flows.

The temperature adjusting zone 51 is a zone for adjusting a surfacetemperature of the sheet-like foamy thermoplastic resin material 6 aextruded from the die 3 to a temperature in a desired temperature range.To be more specific, the sheet-like foamy thermoplastic resin material 6a is heated to a predetermined temperature by heat supplied from thetemperature adjusting medium paths 54. The provision of the temperatureadjusting zone 51 enables adjustment of a temperature in the expandingzone 7 during the expansion, thereby ensuing more stable production ofthe foamed thermoplastic resin sheet 6.

The set temperature is determined depending on a thermoplastic resin anda foaming agent used, and in the case where the thermoplastic resin is acrystalline resin, the temperature is set not lower than thecrystallization point, and not higher than a temperature of thesheet-like foamy thermoplastic resin material 6 a at the outlet of thedie 3.

For example, in the case where a polypropylene resin is used as athermoplastic resin, the temperature of the temperature adjustment zone51 is set to 130° C. to 180° C., and the surface temperature of thesheet-like foamy thermoplastic resin material 6 a is controlled so as tofall in a range of ±2° C. to the foregoing set temperature.

Particularly in the case where the die 3 is a circular die and asheet-like foamy thermoplastic resin material which is obtained bycutting in a lengthwise direction a cylindrical-form foamy thermoplasticresin material extruded from the die 3 is expanded under a reducedpressure, the manufacturing device of a foamed thermoplastic resin sheetpreferably has the temperature adjusting zone 51.

Incidentally, the means for heating the inner walls 57 is not limited tothe temperature adjusting medium paths 54, and there is no specificlimitation on the means, provided that it is capable of adjustingtemperature and keeping the temperature of the inner walls 57 to the settemperature.

Furthermore, since the temperature of the surfaces of the sheet-likefoamy thermoplastic resin material 6 a can be more easily controlled inthe case where the whole surfaces of the sheet-like foamy thermoplasticresin material 6 a are in contact with the inner walls 57, it ispreferable that the gap between the wall surfaces 52 a and 53 a of theinner walls 57 in the temperature adjusting zone 51 is substantiallyequal to W₁, i.e., substantially equal to a thickness of the sheet-likefoamy thermoplastic resin material 6 a extruded from the die 3, and ifpossible, it is more preferable that the gap between the wall surface 52a and 53 a is adjustable. It is, however, not indispensable to make thewhole surfaces of the sheet-like foamy thermoplastic resin material 6 abe in contact with the inner surfaces of the temperature adjusting zone51.

Furthermore, the manufacturing device may be arranged, as shown in FIGS.10(a) and 10(b), so that the molding section 9 including the temperatureadjusting zone 51 may be distanced from the die 3. Here, at least a pairof rolls 55 is preferably provided at an inlet of the temperatureadjusting zone 51 so that the sheet-like foamy thermoplastic resinmaterial 6 a extruded from the die 3 is taken into the temperatureadjusting zone 51 through between the rolls 55. In this manufacturingdevice, a pair of cutters 56 may be provided at the inlet of thetemperature adjusting zone 51, on both sides thereof in the sheet widthdirection as shown in FIG. 10(b), so as to adjust the width of thesheet-like foamy thermoplastic resin material 6 a.

With the arrangement in which the rolls 55 are provided, it is possibleto adjust the thickness of the sheet-like foamy thermoplastic resinmaterial 6 a before the sheet-like foamy thermoplastic resin material 6a enters the vacuum chamber 4, thereby facilitating the sliding of thesheet-like foamy thermoplastic resin material 6 a into the temperatureadjusting zone 51.

The rolls 55 are preferably arranged so that a temperature thereof canbe controlled, and the temperature is appropriately set depending on athermoplastic resin and a foaming agent used, as well as a thickness ofthe foamed thermoplastic resin sheet 6 to be formed. The temperature is,however, preferably set not higher than the temperature of thesheet-like foamy thermoplastic resin material 6 a at the outlet of thedie 3, and not lower than the set temperature of the temperatureadjusting zone 51.

Incidentally, the cutters 56 are not necessarily provided, but provisionof the same is preferable for the following reasons. By cutting, withthe cutters 56, the sheet-like foamy thermoplastic resin material 6 a soas to have a width in accordance with a transport path width in thetemperature adjusting zone 51, sliding of the sheet-like foamythermoplastic resin material 6 a into the temperature adjusting zone 51is facilitated.

[Fourth Embodiment]

The following description will explain still another embodiment of thepresent invention while referring to FIG. 11. The members having thesame structure (function) as those in the above-mentioned embodimentswill be designated by the same reference numerals and their descriptionwill be omitted.

The device for manufacturing the foamed thermoplastic resin sheet 6 maybe arranged as shown in FIGS. 11(a) and 11(b). This manufacturing deviceis equipped with a circular die 61, in the place of the die 3 whichforms into a sheet form the foamy thermoplastic resin material extrudedfrom the extruder 1 through the head part 2. The circular die 61 isintended to process the foamy thermoplastic resin material extruded fromthe extruder 1 through the head part 2 into a cylindrical foamythermoplastic resin material 6 b.

Behind the circular die 61, there is provided a cutter 62 which cuts inthe extrusion direction the cylindrical foamed thermoplastic resinmaterial 6 b extruded into the atmosphere through the circular die 61,thereby forming it into a developed sheet-like foamy thermoplastic resinmaterial 6 a. Therefore, the cylindrical foamy thermoplastic resinmaterial 6 b is cut out by the cutter 62 thereby becoming the sheet-likefoamy thermoplastic resin material 6 a. Then, it is taken into thevacuum chamber 4 by the rolls 55.

Incidentally, the means for cutting out the cylindrical foamythermoplastic resin material 6 b is not limited to the cutter 56, butany means may be applicable provided that it is capable of the foregoingcutting operation.

Furthermore, the aforementioned manufacturing device is provided withthe temperature adjusting zone 51 before the vacuum chamber 4. Thefunction of the temperature adjusting zone 51 is described as above.Further, the foregoing roll 55 and the cutter 56 are not indispensable,though they are provided at the entrance of the temperature adjustingzone 51 so that the arrangement becomes desirable for taking thesheet-like foamy thermoplastic resin material 6 a into the vacuumchamber 4.

In the manufacturing device as described above, the extruder 1, the die3, and the vacuum chamber 4 are provided in a row in a horizontaldirection, but alternatively, they may be disposed so that the extrusiondirection of the die 3 is a downward direction and that the vacuumchamber 4 is disposed on a product in such an extrusion direction.

Here, the following description will explain a result of observation ofcell forms of foamed thermoplastic resin sheets 6 produced by theforegoing manufacturing devices. As a result of the observation, it wasfound that each obtained foamed thermoplastic resin sheet had a foamingratio of 2.5 or more, and cells existing in an interior part whichextended inward from a depth of 20 percent the whole thickness of thefoamed thermoplastic resin sheet from both the surfaces of the sheetrespectively in the thickness direction of the same, and from a depth of15 percent the width of the foamed thermoplastic resin sheet from boththe side surfaces respectively, satisfied the following expressions (1)and (2):

0.5≦D/C≦0.9  (1)

0.5≦E/C≦0.9  (2)

where C represents a mean cell diameter in the thickness direction ofthe foamed thermoplastic resin sheet, D represents a mean cell diameterin the extrusion direction of the same, and E represents a mean celldiameter in the width direction of the same.

EXAMPLE 1

The following description will explain an example of the presentinvention while referring to FIG. 12.

In the present example, a mixture of polypropylene and polyethylene wasused as the thermoplastic resin, and a mixture ratio thereof, i.e.,polypropylene: polyethylene, was set to 70 wt %:30 wt %. Further, as afoaming agent and a foaming assistant, 3.5 parts by weight of a 30 wt %masterbatch (base: polyethylene) of a composite foaming agent in whichsodium bicarbonate/azodicarbonamide/zinc oxide was 9/0.5/0.5 was added.

The manufacturing device shown in FIG. 1 was used to produce the foamedthermoplastic resin sheet 6 herein. In the manufacturing device, asingle-axis 65 mmφ extruder was used as the extruder 1. Set conditionsof the device are shown in Table 1 below.

TABLE 1 DIE TEMPERATURE 175° C.  EXPANDING ZONE TEMPERATURE 60° C.COOLING ZONE TEMPERATURE 30° C. VACUUM CHAMBER PRESSURE 360 mmHg(DIFFERENTIAL PRESSURE) APPARENT FRICTION COEFFICIENT 0.32

A cross section of the foamed thermoplastic resin sheet 6 produced byusing the foregoing manufacturing device was observed and cell diameterswere measured. It was confirmed that the measurement result satisfiedthe aforementioned requirements (the formulas (1) and (2)). Themeasurement result is shown in Table 2. For a control, a resin with thesame composition and the same foaming agent, extruder 1, and die 3 asthose in the example were used, and a foamed thermoplastic resin sheetwas produced by extrusion to an atmospheric pressure. A result ofobservation of a cross section of the obtained foamed thermoplasticresin sheet is shown in the table.

Incidentally, as measured values of each cell, maximum tangent linedistances of the cell in the thickness direction, the extrusiondirection, and the width direction of the foamed thermoplastic resinsheet were used, as shown in FIG. 12.

Furthermore, (mean cell diameter in the extrusion direction of thefoamed thermoplastic resin sheet)/(mean cell diameter in the thicknessdirection of the foamed thermoplastic resin sheet), that is, D/C, wasmeasured by the following method.

First, an area of 20 cm (in the width direction)×20 cm (in the extrusiondirection) was chosen out of the whole area of the sheet excluding 15percent the width of the sheet from both side edges of the sheet,respectively, and at three points in the chosen area, samples havingcross sections parallel with the extrusion direction and the thicknessdirection and samples having cross sections parallel with the widthdirection and the thickness direction were taken out. Then, regardingeach sample, a photomicrograph of a cross section parallel with theextrusion direction in a region whose depth from each surface of thesheet exceeds 20 percent the whole thickness of the foamed thermoplasticresin sheet was taken. Regarding more than half of cells in a 1 mm²square region in each photomicrograph, c (diameter in the thicknessdirection) and d (diameter in the extrusion direction) were measured inthe manner shown in FIG. 12. Then, from c₁, c₂, . . . c_(n) and d₁, d₂,. . . d_(n) in all the regions thus obtained, C and D which are meanvalues of c and d, respectively, were found, and further, D/C wasobtained. Here, n satisfies n≧30.

Furthermore, (mean cell diameter in the width direction of the foamedthermoplastic resin sheet)/(mean cell diameter in the thicknessdirection of the foamed thermoplastic resin sheet), i.e., E/C wasmeasured by the following method.

Regarding each of the foregoing three samples, a photomicrograph of across section parallel with the width direction in a region whose depthfrom each surface of the sheet exceeds 20 percent the whole thickness ofthe foamed thermoplastic resin sheet was taken. Regarding more than halfof cells in a 1 mm² square region in each photomicrograph, c (diameterin the thickness direction) and e (diameter in the width direction) weremeasured in the manner shown in FIG. 12. Then, from c₁, c₂, . . . c_(n)and e₁, e₂, . . . e_(n) in all the regions thus obtained, C and E whichare mean values of c and e, respectively, were found, and further E/Cwas obtained. Here, n satisfies n≧30.

Smoothness of surfaces of the foamed thermoplastic resin sheet 6 wasevaluated by *center line average height Ra indicating surface roughnessspecified by JIS B0601. The center line average height Ra was measuredunder conditions of a cut-off value of 0.8 mm, a measured length of 10mm, and a driving speed of 0.3 mm/S, and the measured result is anaverage of measured values at 5 points.

TABLE 2 MAG- NI- FICA- SUR- TION FACE THICKNESS (DIA- CELL FORM ROUGH-(mm) METER) D/C E/C NESS EXAMPLE 6.8 5.4 0.7 0.8 ∘ CONTROL 2.8 2.7 1.41.3 Δ Center line average height Ra (mm) ∘: Ra ≦ 0.4 Δ: 0.4 < Ra ≦ 0.8x: 0.8 < Ra C: mean cell diameter in the thickness direction D: meancell diameter in the extrusion direction E: mean cell diameter in thewidth direction

It was confirmed that the foamed thermoplastic resin sheet 6 of theexample having the cell form as described above had a high foaming ratioand a great thickness.

The foamed thermoplastic resin sheet manufacturing device of the presentinvention may be arranged so as to further comprise driving means formoving the movable wall in the directions, wherein (i) the die has anoutlet part which is long in the width direction of the foamythermoplastic resin material, (ii) each of the walls in pair in thefacing-wall section is the movable wall, and (iii) the driving meansmoves the movable walls to positions which are at equal distancesrespectively from a reference position which is a central position ofthe foamy thermoplastic resin material at the outlet part in thethickness direction of the resin material.

With the foregoing arrangement, driven by the driving means, the movablewalls move to positions which are at equal distances respectively from areference position which is a central position, in the resin materialthickness direction, of the foamy thermoplastic resin material at theoutlet part of the die.

Therefore, respective position relations between the outlet part of thedie and the walls in pair of the facing-wall section coincide with eachother, thereby ensuring that the formation of the foamed thermoplasticresin sheet between the walls in the vacuum chamber is smoothly carriedout. Besides, on both sides to the foamed thermoplastic resin sheet, thereduced pressure in the vacuum chamber is stably maintained.

Furthermore, the foamed thermoplastic resin sheet manufacturing deviceof the present invention may be further arranged so as to comprisetemperature adjusting means for adjusting a temperature of the foamythermoplastic resin material in the vacuum chamber.

With the foregoing arrangement, immediately after the expansion of thefoamy thermoplastic resin material under a reduced pressure in thevacuum chamber, a temperature of the foamed thermoplastic resin sheetobtained is adjusted to a desired temperature by the temperatureadjusting means. As a result, the foamed thermoplastic resin sheet canbe kept in the highly foaming state under the reduced pressure.

Furthermore, the foamed thermoplastic resin sheet manufacturing deviceof the present invention may be further arranged so that (i) thetemperature adjusting means includes a cooling medium path provided ininner walls of the vacuum chamber, and (ii) at least a part of the foamythermoplastic resin material is in contact with the inner walls of thevacuum chamber.

With the foregoing arrangement, the highly foaming state of the foamedthermoplastic resin sheet can be more surely maintained.

Furthermore, the foamed thermoplastic resin sheet manufacturing deviceof the present invention may be further arranged so that the vacuumchamber includes an expanding zone for expanding the foamy thermoplasticresin material and a cooling zone for cooling the foamed thermoplasticresin sheet obtained by expansion of the foamy thermoplastic resinmaterial under a reduced pressure.

According the foregoing arrangement, the foamy thermoplastic resinmaterial expands in the expanding zone thereby becoming the foamedthermoplastic resin sheet, and thereafter the sheet is cooled in thecooling zone. As a result, the foamed thermoplastic resin sheet is keptin the highly foaming state under the reduced pressure.

Furthermore, the foamed thermoplastic resin sheet manufacturing deviceof the present invention may be further arranged so as to comprisetemperature adjusting means for adjusting a temperature of the foamythermoplastic resin material in the expanding zone.

With the foregoing arrangement, immediately after the expansion of thefoamy thermoplastic resin material under a reduced pressure in theexpanding zone, a temperature of the foamed thermoplastic resin sheetobtained is adjusted to a desired temperature by the temperatureadjusting means. As a result, the foamed thermoplastic resin sheet canbe kept in the highly foaming state under the reduced pressure.

Furthermore, the foamed thermoplastic resin sheet manufacturing deviceof the present invention may be arranged so that (i) the temperatureadjusting means includes a cooling medium path provided in inner wallsof the vacuum chamber in the expanding zone, and (ii) at least a part ofthe foamy thermoplastic resin material is in contact with the innerwalls of the vacuum chamber in the expanding zone.

With the foregoing arrangement, the highly foaming state of the foamedthermoplastic resin sheet can be more surely maintained.

Furthermore, the foamed thermoplastic resin sheet manufacturing deviceof the present invention may be arranged so as to comprise a coolingmedium path provided in inner walls of the vacuum chamber in the coolingzone, wherein at least a part of the foamed thermoplastic resin sheet isin contact with the inner walls of the vacuum chamber in the coolingzone.

With the foregoing arrangement, the cooling zone for cooling the foamedthermoplastic resin sheet can be easily realized, while the foamedthermoplastic resin sheet can be efficiently cooled. Incidentally, thefoamed thermoplastic resin sheet may be partially brought into contactwith the inner walls of the vacuum chamber, but it is preferable thatthe whole surfaces of the foamed thermoplastic resin sheet are broughtinto contact therewith.

Furthermore, the foamed thermoplastic resin sheet manufacturing deviceof the present invention may be arranged so that the die has an outletpart which is long in the width direction of the foamy thermoplasticresin material and projects to inside of the vacuum chamber.

Usually, a part of the foamy thermoplastic resin material extruded fromthe outlet part of the die tends to get into a crack between the movablewall and the die in a state in which the gap between the walls in pairof the facing-wall section which face each other is narrowed, therebyhindering transport of the foamy thermoplastic resin material. However,this can be avoided by the foregoing arrangement wherein the outlet partof the die projects into the vacuum chamber.

Thus, as described above, in the foamed thermoplastic resin sheetmanufacturing device of the present invention, the vacuum chamber isarranged so that temperature is more easily adjusted as compared withthe vacuum chamber of the conventional device. Therefore, a foamedthermoplastic resin sheet with its foaming state under a reducedpressure well maintained can be obtained. As a result, a foamedthermoplastic resin sheet which fully enjoys an effect of cell growthdue to pressure reduction thereby having the cells therein grown up inthe thickness direction of the sheet, and has a good appearance.

Furthermore, the foamed thermoplastic resin sheet manufacturing deviceof the present invention may be arranged so that at least a part ofwalls enclosing the vacuum chamber is made of a material having aplurality of vacuum ports, and evacuation of the vacuum chamber iscarried out through the material having vacuum ports.

According to the foregoing arrangement, at least a part of wallsenclosing the vacuum chamber is made of a material having a plurality ofvacuum ports, and evacuation of the vacuum chamber is carried outthrough the material. Therefore, even if, for example, a part of thevacuum ports is clogged up with the foamy thermoplastic resin material,evacuation of the vacuum chamber is ensured. Incidentally, examples ofthe materials having a plurality of vacuum ports include a member onwhich a number of fine slits with a very small width each are formed.

Furthermore, the foamed thermoplastic resin sheet manufacturing deviceof the present invention may be arranged so that the material havingvacuum ports is a porous material.

With the foregoing arrangement, the aforementioned accident that thefoamy thermoplastic resin material clogs up a part of the vacuum portsthereby hindering the evacuation of the vacuum chamber, for example, canbe more surely avoided.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A manufacturing device of a foamed thermoplasticresin sheet, comprising: an extruder for melting and kneading athermoplastic resin and a foaming agent to form a melted/kneadedmixture, and extruding the melted/kneaded mixture; a die provided at afront end of said extruder, for forming the melted/kneaded mixture intoa sheet form foamy thermoplastic resin material; a vacuum chamber inwhich the foamy thermoplastic resin material extruded through said dieexpands under a reduced pressure; a facing-wall section composed of apair of walls of said vacuum chamber which face each other in athickness direction of the foamy thermoplastic resin material, at leastone of the walls being a movable wall that is movable in directions inwhich said walls approach and separate each other, and is movable whilestaying in a parallel state with the other wall of said pair of walls,and wherein at least one of said pair of walls has vacuum ports with adiameter of 20 mm or smaller; a sealing member provided at least on saidmovable wall in said facing-wall section, at an outlet of said vacuumchamber, so as to seal a space between said wall and a foamedthermoplastic resin sheet coming out of said vacuum chamber, the foamedthermoplastic resin sheet being obtained as a result of expansion of thefoamy thermoplastic resin material under a reduced pressure; and drivingmeans for driving said movable wall so as to control a space in saidfacing-wall section, depending on a thickness of the sheet form foamythermoplastic resin material changed by the expansion caused by thereduced pressure.
 2. The manufacturing device as set forth in claim 1,wherein said driving means is a means for moving said movable wall insaid directions, and wherein: said die has an outlet part which is longin the width direction of the foamy thermoplastic resin material; eachof said walls in said pair of walls in said facing-wall section is amovable wall; and said driving means moves said movable walls topositions which are at equal distances from a reference position, thereference position being a central position of the foamy thermoplasticresin material at said outlet part in the thickness direction of thefoamy thermoplastic resin material.
 3. The manufacturing device as setforth in claim 1, further comprising temperature adjusting means foradjusting a temperature of said foamy thermoplastic resin material insaid vacuum chamber.
 4. The manufacturing device as set forth in claim3, wherein: said temperature adjusting means includes a cooling mediumpath provided in inner walls of said vacuum chamber; and at least a partof the foamy thermoplastic resin material is in contact with the innerwalls of said vacuum chamber.
 5. The manufacturing device as set forthin claim 1, wherein said vacuum chamber includes an expanding zone forexpanding the foamy thermoplastic resin material, and a cooling zone,provided behind said expanding zone, for cooling the foamedthermoplastic resin sheet obtained by expansion of the foamythermoplastic resin material under a reduced pressure.
 6. Themanufacturing device as set forth in claim 5, further comprisingtemperature adjusting means for adjusting a temperature of said foamythermoplastic resin material in said expanding zone.
 7. Themanufacturing device as set forth in claim 6, wherein: said temperatureadjusting means includes a cooling medium path provided in inner wallsof said vacuum chamber in said expanding zone; and at least a part ofthe foamy thermoplastic resin material is in contact with the innerwalls of said vacuum chamber in said expanding zone.
 8. Themanufacturing device as set forth in claim 5, further comprising acooling medium path provided in inner walls of said vacuum chamber insaid cooling zone, wherein at least a part of the foamed thermoplasticresin sheet is in contact with the inner walls of said vacuum chamber insaid cooling zone.
 9. The manufacturing device as set forth in claim 1,wherein said die has an outlet part which is long in the width directionof the foamy thermoplastic resin material, said outlet part projectingto inside of said vacuum chamber.
 10. The manufacturing device as setforth in claim 1, wherein: at least a part of walls enclosing saidvacuum chamber is made of a material having a plurality of vacuum ports;and evacuation of said vacuum chamber is carried out through saidmaterial having vacuum ports.
 11. The manufacturing device as set forthin claim 10, wherein said material having vacuum ports is a porousmaterial.
 12. The manufacturing device as set forth in claim 11, whereinsaid porous material is a porous electrocast shell.
 13. Themanufacturing device as set forth in claim 11, wherein said porousmaterial is a sintered alloy.
 14. The manufacturing device as set forthin claim 11, wherein the vacuum port of said porous material has adiameter of not more than 100 μm.
 15. The manufacturing device as setforth in claim 14, wherein said porous material is a porous electrocastshell.
 16. The manufacturing device as set forth in claim 14, whereinsaid porous material is a sintered alloy.
 17. The manufacturing deviceas set forth in claim 1, wherein at least a part of surfaces of wallsenclosing said vacuum chamber contains protuberances, so as to decreasean area of the sheet in contact with the wall surfaces when the sheetsent from said die to said vacuum chamber is transported through saidvacuum chamber.
 18. The manufacturing device as set forth in claim 17,wherein the protuberances of the wall surfaces are plated withpolytetrafluoroethylene (Teflon).
 19. The manufacturing device as setforth in claim 17, wherein the protuberances have curving surfaces. 20.The manufacturing device as set forth in claim 17, wherein: at least apart of the walls enclosing said vacuum chamber is made of a porousmaterial; and evacuation of said vacuum chamber is carried out throughsaid porous material.
 21. The manufacturing device as set forth in claim1, further comprising: adjusting means for reducing pressure in thevacuum chamber so as to become lower than the atmospheric pressure bynot less than 100 mmHg and not more than 700 mmHg.
 22. The manufacturingdevice as set forth in claim 17, further comprising: adjusting means forreducing pressure in the vacuum chamber so as to become lower than theatmospheric pressure by not less than 100 mmHg and not more than 700mmHg.
 23. The manufacturing device as set forth in claim 17, wherein theprotuberances are formed so that an area of the surfaces of walls incontact with the foamy thermoplastic resin material should be not lessthan 10 percent and not more than 80 percent of an area in the casewhere the surfaces of walls do not have protuberances.
 24. Amanufacturing device of a foamed thermoplastic resin sheet, comprising:an extruder for melting and kneading a thermoplastic resin and a foamingagent to form a melted/kneaded mixture, and extruding the melted/kneadedmixture; a circular die provided at a front end of said extruder; meansfor cutting out a cylindrical foamy thermoplastic resin materialextruded from said circular die, so as to form the cylindrical foamythermoplastic resin material into a sheet form; a vacuum chamber inwhich the sheet form foamy thermoplastic resin material expands under areduced pressure; and a facing-wall section composed of a pair of wallsof said vacuum chamber which face each other in a thickness direction ofthe foamy thermoplastic resin material, at least one of the walls beinga movable wall that is movable in directions in which said wallsapproach and separate each other, and is movable while staying in aparallel state with the other wall of said pair of walls.